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Recent Developments in MCNP and MCNPX

H. Grady HughesLos Alamos National Laboratory

Uncertainty Assessment in Computational DosimetryBologna, Italy

October 8 –10, 2007


Abstract

New features of MCNP and MCNPX will be discussed with particular attention to the forthcoming release of MCNP5 and to the ongoing MCNP/MCNPX unification project. One particular new feature for electron transport, the detailed straggling sampling (first made available in version 1.40 of MCNP5) will be described, and its effect on the calculation of pulse-height spectra in a mini-TEPC simulation will be reported.


New Features in MCNP5 version 1.40

• Stochastic geometry

• Source entropy estimation and plotting (for criticality)

• Fission neutron multiplicity

• New electron energy-loss straggling logic

• Positron sources

• Lethargy plots for energy-dependent tallies

• Mesh tally plotter

• Input conveniences


Stochastic Geometry

• On-the-fly random translations of embedded universes in lattice

• Developed for pebble bed reactors.

• Could model other amorphous materials

• Medical physics – Alveoli, sinuses, bone

marrow, … ?

Fuel kernel displaced randomly within lattice element each time that particle enters

Image of the stochasticgeometry of fuel kernels from MCNP5 plotter

Forrest Brown, “Monte Carlo Methods & MCNP Code Development” Monte Carlo 2005, Chattanooga, TN.



New Landau Straggling Logic

• Bin-centered (MCNP) logic– One sample per energy step– Emphasis is on remaining (mostly) within the energy group– DBCN 17J 0 (still the default)

• Nearest-group boundary (ITS) logic– One sample per energy step– Emphasis is on completing a prescribed energy step– DBCN 17J 1 (often preferred)

• Detailed straggling logic– One sample per angular sub-step– Sampling function is specific to particle energy and step length– DBCN 17J 2 (new with MCNP5 1.40)


Landau Straggling Example

• Three representations of the same geometry

– 10 MeV electrons

– 15 mm water slab

– No angular deflection

– Substep = 1.364 mm


3 4 5 6 7 8Transmitted Electron Energy (MeV)

1×10-3

1×10-2

1×10-1

1×100

1×101

Num

ber

per

MeV

per

Sou

rce

Ele

ctro

n 15 mm cell1 mm cells0.1 mm cells

MCNP (bin-centered) Straggling Logic10 MeV electrons after 15 mm water, no angular deflection



1×10-3

1×10-2

1×10-1

1×100

1×101

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ber

per

MeV

per

Sou

rce

Ele

ctro


ITS (nearest-group-boundary) Straggling Logic10 MeV electrons after 15 mm water, no angular deflection



1×10-3

1×10-2

1×10-1

1×100

1×101

Num

ber

per

MeV

per

Sou

rce

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ctro


Step-Specific Straggling Logic10 MeV electrons after 15 mm water, no angular deflection


New Features in MCNP5 version 1.50

• Pulse-height tally variance reduction– Including DXTRAN– Memory management for threading

• Generation of temperature-specific cross sections– Enhancement to MAKXSF code– Doppler broaden resolved resonance data to higher temperature– Interpolate unresolved resonance data and S( , ) data

• Lattice memory improvements

• Improved annihilation gamma treatment

• Long file names (256 characters)


Lattice Memory Improvements

• Increase limit on number of voxels from ~20 Million to ~200+ Million.

• Reduce startup times from hours or days to a few hours.

• Windows OS limit of 2 Gigabytes of Memory per program. (Use 64 bit chip & OS)

0

100

200

300

400

500

600

700

800

900

0 50 100 150 200 250 300 350 400

Millions of Voxel Cells

Meg

abyt

es o

f Mem

ory MCNP5_RSICC_1.20

MCNP5 VIP Modifications

Goorley, “Issues Related to the use of MCNP code for an

Extremely Large Voxel Model VIP-MAN” Monte Carlo 2005.


Selected Recent MCNPX Features

• Version 2.6.A– Transmutation using CINDER90– Proton step size control– New S( , ) scattering law– STOP card– Long file names (40 characters)

• Version 2.6.B– CEM INC model upgrade (2K 03)– Gravitational effects for neutrons– Improved high-energy photon emission data (PHTLIB)


More MCNPX Features

• Version 2.6.C– Delayed neutrons and gammas– Spherical weight windows– Photon tally tagging

• Version 2.6.D– Coupled space-energy-time weight windows– Activation neutrons and gammas– Background radioactive sources– Muon capture physics– Heavy ion transport


New S( , ) Data

2.6.C/new S( , ) file

2.5.0/lwtr.01t

Neutron flux in the water


CEM Upgrade

2.6.C neutrons

2.6.C protons2.5.0 protons

2.5.0 neutrons

Secondary particle productionFrom 1-GeV protons onto 56Fe


Photon Tally Tagging

27Si

26Si

27Al

26Al

Deexcitation gammas from 28Si


MCNP/MCNPX Merger Project

• Goal – Combine all features of MCNPX and MCNP5 into a single code, to be released as MCNP6.

• Principal developers– H. G. Hughes, G. W. McKinney, J. S. Hendricks, J. T. Goorley,

S. J. Pollack, R. E. Prael

• Support– FY07 – 2.5 FTE– FY08 – at least 1.25 FTE

• Expected delivery – end of FY08.


MCNP/MCNPX Merger Project: Overview

• ~1000 subroutines in 9 directories• ~500 new subroutines in 8 sub-directories• ~400 subroutines in the MCNP6 …/src/ directory

• ~400 subroutines in the MCNPX …/mcnpf/ directory• 55 new MCNPX subroutines• ~200 subroutines nearly same as MCNP6• ~150 subroutines require careful merging

• Phased approach with building & testing after each step


MCNP/MCNPX Merger Project: PlanPhase 1 Phase 5

Move X variables to 6. MCRUN

Reconcile particles, etc. (particle transport)

Phase 2 Phase 6

First half of IMCN MCRUN

(card reading) (sources and tallies)

Phase 3 Phase 7

Second half of IMCN PLOT

(geometry, tallies, materials) (tallies with geometry)

Phase 4 Phase 8

XACT MCPLOT

(proton lib., heating) (mesh tallies)



Integral Numbers and Energies

• Total energies : In Out NetPhotons 0.1788 0.1788 9.2×10-7 MeVElectrons 3.846×10-4 3.847×10-4 -1.6×10-7 MeV

• Heating estimators: F6:P F8:P,Ecell 1 0.707 0.591 MeV/gmcell 41 0.707 0.619 MeV/gmcell 51 0.694 0.535 MeV/gmcell 61 0.605 0.353 MeV/gm

• Numbers for cell 1: Photons ElectronsIn 0.271 1.69×10-3

Out 0.271 1.69×10-3

Generated 4.56×10-8 5.47×10-5

Lost 4.10×10-8 5.49×10-5



10-5

10-4

10-3

10-2

Energy (MeV)

10-3

10-2

10-1

100

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102

Cou

nts

per

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per

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phot

on

MCNP5, DBCN(18) = 1.MCNP5, DBCN(18) = 2.

Cs-137 Source


10-5

10-4

10-3

10-2

Energy (MeV)

10-3

10-2

10-1

100

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102

Cou

nts

per

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on

MCNP5, DBCN(18) = 1.MCNP5, DBCN(18) = 2.FLUKAMCNPXfrom B. Grosswendt

Cs-137 Source


The End

• Comments ?

• Questions ?

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